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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 19:33:14 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-07 19:33:14 +0000 |
commit | 36d22d82aa202bb199967e9512281e9a53db42c9 (patch) | |
tree | 105e8c98ddea1c1e4784a60a5a6410fa416be2de /third_party/rust/proc-macro-hack/README.md | |
parent | Initial commit. (diff) | |
download | firefox-esr-36d22d82aa202bb199967e9512281e9a53db42c9.tar.xz firefox-esr-36d22d82aa202bb199967e9512281e9a53db42c9.zip |
Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'third_party/rust/proc-macro-hack/README.md')
-rw-r--r-- | third_party/rust/proc-macro-hack/README.md | 155 |
1 files changed, 155 insertions, 0 deletions
diff --git a/third_party/rust/proc-macro-hack/README.md b/third_party/rust/proc-macro-hack/README.md new file mode 100644 index 0000000000..b72b8ef9dd --- /dev/null +++ b/third_party/rust/proc-macro-hack/README.md @@ -0,0 +1,155 @@ +Procedural macros in expression position +======================================== + +[<img alt="github" src="https://img.shields.io/badge/github-dtolnay/proc--macro--hack-8da0cb?style=for-the-badge&labelColor=555555&logo=github" height="20">](https://github.com/dtolnay/proc-macro-hack) +[<img alt="crates.io" src="https://img.shields.io/crates/v/proc-macro-hack.svg?style=for-the-badge&color=fc8d62&logo=rust" height="20">](https://crates.io/crates/proc-macro-hack) +[<img alt="docs.rs" src="https://img.shields.io/badge/docs.rs-proc--macro--hack-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" height="20">](https://docs.rs/proc-macro-hack) +[<img alt="build status" src="https://img.shields.io/github/actions/workflow/status/dtolnay/proc-macro-hack/ci.yml?branch=master&style=for-the-badge" height="20">](https://github.com/dtolnay/proc-macro-hack/actions?query=branch%3Amaster) + +<table><tr><td><hr> +<b>Note:</b> <i>As of Rust 1.45 this crate is superseded by native support for +#[proc_macro] in expression position. Only consider using this crate if you care +about supporting compilers between 1.31 and 1.45.</i> +<hr></td></tr></table> + +Since Rust 1.30, the language supports user-defined function-like procedural +macros. However these can only be invoked in item position, not in statements or +expressions. + +This crate implements an alternative type of procedural macro that can be +invoked in statement or expression position. + +This approach works with any Rust version 1.31+. + +## Defining procedural macros + +Two crates are required to define a procedural macro. + +### The implementation crate + +This crate must contain nothing but procedural macros. Private helper +functions and private modules are fine but nothing can be public. + +[» example of an implementation crate][demo-hack-impl] + +Just like you would use a #\[proc_macro\] attribute to define a natively +supported procedural macro, use proc-macro-hack's #\[proc_macro_hack\] +attribute to define a procedural macro that works in expression position. +The function signature is the same as for ordinary function-like procedural +macros. + +```rust +use proc_macro::TokenStream; +use proc_macro_hack::proc_macro_hack; +use quote::quote; +use syn::{parse_macro_input, Expr}; + +#[proc_macro_hack] +pub fn add_one(input: TokenStream) -> TokenStream { + let expr = parse_macro_input!(input as Expr); + TokenStream::from(quote! { + 1 + (#expr) + }) +} +``` + +### The declaration crate + +This crate is allowed to contain other public things if you need, for +example traits or functions or ordinary macros. + +[» example of a declaration crate][demo-hack] + +Within the declaration crate there needs to be a re-export of your +procedural macro from the implementation crate. The re-export also carries a +\#\[proc_macro_hack\] attribute. + +```rust +use proc_macro_hack::proc_macro_hack; + +/// Add one to an expression. +/// +/// (Documentation goes here on the re-export, not in the other crate.) +#[proc_macro_hack] +pub use demo_hack_impl::add_one; +``` + +Both crates depend on `proc-macro-hack`: + +```toml +[dependencies] +proc-macro-hack = "0.5" +``` + +Additionally, your implementation crate (but not your declaration crate) is +a proc macro crate: + +```toml +[lib] +proc-macro = true +``` + +## Using procedural macros + +Users of your crate depend on your declaration crate (not your +implementation crate), then use your procedural macros as usual. + +[» example of a downstream crate][example] + +```rust +use demo_hack::add_one; + +fn main() { + let two = 2; + let nine = add_one!(two) + add_one!(2 + 3); + println!("nine = {}", nine); +} +``` + +[demo-hack-impl]: https://github.com/dtolnay/proc-macro-hack/tree/master/demo-hack-impl +[demo-hack]: https://github.com/dtolnay/proc-macro-hack/tree/master/demo-hack +[example]: https://github.com/dtolnay/proc-macro-hack/tree/master/example + +## Limitations + +- Only proc macros in expression position are supported. Proc macros in pattern + position ([#20]) are not supported. + +- By default, nested invocations are not supported i.e. the code emitted by a + proc-macro-hack macro invocation cannot contain recursive calls to the same + proc-macro-hack macro nor calls to any other proc-macro-hack macros. Use + [`proc-macro-nested`] if you require support for nested invocations. + +- By default, hygiene is structured such that the expanded code can't refer to + local variables other than those passed by name somewhere in the macro input. + If your macro must refer to *local* variables that don't get named in the + macro input, use `#[proc_macro_hack(fake_call_site)]` on the re-export in your + declaration crate. *Most macros won't need this.* + +- On compilers that are new enough to natively support proc macros in expression + position, proc-macro-hack does not automatically use that support, since the + hygiene can be subtly different between the two implementations. To opt in to + compiling your macro to native `#[proc_macro]` on sufficiently new compilers, + use `#[proc_macro_hack(only_hack_old_rustc)]` on the re-export in your + declaration crate. + +[#10]: https://github.com/dtolnay/proc-macro-hack/issues/10 +[#20]: https://github.com/dtolnay/proc-macro-hack/issues/20 +[`proc-macro-nested`]: https://docs.rs/proc-macro-nested + +<br> + +#### License + +<sup> +Licensed under either of <a href="LICENSE-APACHE">Apache License, Version +2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option. +</sup> + +<br> + +<sub> +Unless you explicitly state otherwise, any contribution intentionally submitted +for inclusion in this hack by you, as defined in the Apache-2.0 license, shall +be dual licensed as above, without any additional terms or conditions. +</sub> |